U.S. Pat. No. 4,481,442, ALBRECHT, HOFMANN, KLEIN and PANOFSKI, assigned to the assignee of the present application, the disclosure of which is hereby incorporated by reference, discloses a glass vessel for use as a compact fluorescent tube, and a method to make such a vessel. A generally straight glass tube forms the starting material. The portions of the tube which are to be bent are heated to softening temperature. By then bending the unheated tube portions against each other, a 180.degree. bend is formed. The bent portion is introduced into a mold and compressed air is blown into the ends of the tube, so that the bent portion will then receive the shape of the mold. The bent portions form, essentially, right-angle corners which are used as cold spots to ensure optimal operation of the lamp by appropriately controlling the gas pressure within the lamp during operation thereof.
U.S. Pat. Nos. 4,288,239 and 4,319,162 describe a method to manufacture discharge lamps which are formed with multiple bends. An essentially straight glass tube is heated, at appropriate regions, until it softens and the non-heated portions are then bent about a mandrel or shaping form or die so that the discharge vessel will receive the desired shape. The bends form generally curved portions and do not have sharp corners which can provide cold spots.
The wall thickness of the glass tube, it has been found, decreases at the essentially square corners formed by the bending and subsequent molding in order to obtain the shape of the vessel described in the referenced U.S. Pat. No. 4,481,442. Bending the tube alone already decreases the wall thickness and when making the essentially square corners, the wall thickness decreases further. It has been found that due to the thin remaining glass wall, lamps of this type are subject to breakage at the bent corners.
Breakage at the bent corners can be prevented by making the glass of the glass tube already with a sufficient thickness so that, after forming the 180.degree. bend, a predetermined minimum thickness will remain even at the bent corners. This, however, substantially increases the weight of the overall discharge vessel, and its cost.